Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas and is not economical.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). Those organic molecules containing only carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen known as oxygenates may be formed during the Fischer-Tropsch process. Hydrocarbons having carbons linked in a straight chain are known as aliphatic hydrocarbons that may include paraffins and/or olefins. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reactor. In a batch process, the reactor is closed to introduction of new feed and exit of products. In a continuous process, the reactor is open, with an inflow containing feed, termed a feed stream, passed into the reactor and an outflow containing product, termed a product stream, passed out of the reactor.
Typically the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Thus, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, as well as heavier middle distillates. Hydrocarbon waxes may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel it is desirable to obtain primarily hydrocarbons that are liquids and waxes, that are nongaseous hydrocarbons (e.g. C5+ hydrocarbons).
Typically, in the Fischer-Tropsch synthesis, the product spectra can be described by likening the Fischer-Tropsch reaction to a polymerization reaction with a Shultz-Flory chain growth probability (α) that is independent of the number of carbon atoms in the lengthening molecule. α is typically interpreted as the ratio of the mol fraction of the Cn+1 product to the mol fraction of the Cn product. A value of α of at least 0.72 is desirable for producing high carbon-length hydrocarbons, such as those of diesel fractions.
The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification).
Cobalt metal is particularly desirable in catalysts used in converting natural gas to heavy hydrocarbons suitable for the production of diesel fuel. Alternatively, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but the disadvantage of a water-gas shift activity. Ruthenium has the advantage of high activity but is quite expensive. Consequently, although ruthenium is not the economically preferred catalyst for commercial Fischer-Tropsch production, it is often used in low concentrations as a reduction promoter, particularly for cobalt based Fischer-Tropsch catalysts.
Catalysts often further employ a promoter in conjunction with the principal catalytic metal. A promoter typically improves a measure of the performance of a catalyst, such as productivity, lifetime, selectivity, reducibility, or regenerability. Further, in addition to the catalytic metal, a Fischer-Tropsch catalyst often includes a support material. The support is typically a porous material that provides mechanical strength and a high surface area, in which the active metal and promoter(s) can be deposited. Catalyst supports for catalysts used in Fischer-Tropsch synthesis of hydrocarbons have typically been refractory oxides (e.g., silica, alumina, titania, thoria, zirconia or mixtures thereof, such as silica-alumina). In particular, y-alumina is a popular support for Fischer-Tropsch catalysts.
The method of preparation of a catalyst may influence the performance of the catalyst in the Fischer-Tropsch reaction. In a common method of loading a Fischer-Tropsch metal to a support, the support is impregnated with a solution containing a dissolved metal-containing compound. The metal may be impregnated in a single impregnation, drying and calcinations step or in multiple steps. When a promoter is used, an impregnation solution may further contain a promoter-containing compound. After drying the support, the resulting catalyst precursor is calcined, typically by heating in an oxidizing atmosphere, to decompose the metal-containing compound to a metal oxide. When the catalytic metal is cobalt, the catalyst precursor is then typically reduced in hydrogen to convert the oxide compound to reduced “metallic” metal. When the catalyst includes a promoter, the reduction conditions may cause reduction of the promoter or the promoter may remain as an oxide compound. Despite the vast knowledge of preparation techniques, there is ongoing effort for improving methods of catalyst preparation.
Kraum and Baems (Applied Catalyst A: General 1999, 186, p. 189) describe studies of the performance of titania-supported catalysts prepared with various cobalt compounds, including cobalt (III) acetyl acetonate, cobalt acetate, cobalt oxalate, cobalt nitrate, and cobalt-EDTA. The nominal cobalt loading was 12 wt %. They concluded that “the type of cobalt precursor used for the preparation of TiO2 supported catalysts affects activity and chain growth probability under fixed FTS [Fischer-Tropsch synthesis] conditions”. In particular, they concluded that “For titania-supported catalysts, the use of oxalate, acetate and acetyl acetonate as cobalt precursors resulted in higher activity compared with the reference catalyst prepared from nitrate.” However, the authors further concluded that “the range of chain growth probabilities increased in the following order, cobalt (III) acetyl acetonate (α=0.71)<cobalt acetate (α=0.74)<cobalt (III) acetyl acetonate, cobalt oxalate, cobalt nitrate, cobalt-EDTA (α=0.82–0.84).” The authors further reported that “On adding 0.1 wt % Ru to the catalyst made from cobalt (III) acetyl acetonate, α increased from 0.71 to 0.80.” Thus although the titania-supported catalysts prepared with cobalt acetate and cobalt (III) acetyl acetonate had higher activity, they were also less selective to heavier hydrocarbons.
Fan and Fujimoto. (Chemistry Letters 1999, p. 343) describe studies of the performance of silica supported catalysts prepared with various cobalt compounds, including cobalt nitrate, cobalt acetate, cobalt chloride, and combinations of cobalt nitrate and cobalt acetate. Catalysts made by co-impregnating and sequentially impregnating cobalt nitrate and cobalt acetate were studied. The nominal cobalt loading was 10 wt. %. The authors disclosed that ‘the order of catalytic activity was Co (N/A)>Co (N+A), Co (A/N) and Co (N)>>Co (A).” where Co (N/A) indicates nitrate impregnated before acetate and Co (N+A) indicates nitrate and acetate co-impregnated. Reported chain growth probabilities were similar for Co (N/A), Co (N+A), Co (A/N), and Co (N) (α=0.84–0.86).
A comparison of the results of Fan and Fujimoto with those of Kraum and Baems suggests that the variation in catalyst performance with various cobalt compounds is dependent on the nature of the support. Further, it is known that cobalt interacts more strongly with alumina than silica. Thus, there remains a need for an improved method of preparing a supported cobalt catalyst where the support includes alumina.
Further, it is well known that the use of noble metals improves the performance of cobalt-based Fischer-Tropsch catalysts. However, the use of noble metal promoters has the disadvantage of increasing the cost of Fischer-Tropsch catalysts. Thus, there remains a need for a method of making a cobalt-based Fischer-Tropsch catalyst that involves the use of reduced amounts of noble metal promoters.